CMC is an anionic water-soluble polymer that is used in a variety of industrial and consumer applications. CMC generally is obtained from cellulose (e.g., from cotton linters, wood pulp, or other cellulosic sources) by substitution of at least a portion of the hydroxyl groups for carboxymethyl ether groups and may have a degree of substitution (“DS”) of as high as 3.0, and a molecular weight ranging from about 30,000 to 1,000,000 Daltons.
Due to its ability to impart rheological properties, improve water retention, and improve the efficiency of optical brighteners, CMC is a popular additive in the paper and board industry, especially for coating, sizing, and base web manufacturing. In paper and board coating processes, CMC is added as a pre-dissolved solution (typically having 6 to 12 wt % CMC) into the coating makedown composition, which usually includes inorganic pigments, organic binders, and additives such as dispersants, optical brighteners, crosslinkers, lubricants, and dyes. The coating compositions usually have a solids content between 63 and 70 wt %, depending on the type of coating composition and the type of coating machinery being used; however, coating compositions having both higher and lower solids contents also are used in practice.
Not wishing to be bound by any theory, it is believed that coating compositions having high solids contents are preferable to improve quality and productivity, for example, by improving coating coverage and gloss while decreasing the amount of water that must be evaporated (i.e., thereby reducing drying energy) from coatings. When increasing the target solids content of the coating composition, higher solids contents also are required for the solutions of raw materials used to prepare the coating composition.
The existing solutions of pre-dissolved CMC having a low solids concentration generally will not enable formulation of a high solids content coating composition (e.g. such as 70 wt %), and typically limit the amount of CMC that can be used in the coating composition required for paper coating. Although dry addition of CMC to a high solids pigment slurry could resolve these issues, mills would need to have handling systems for powders, adequate manpower to add the powder by hand, or a mixing system capable of imparting sufficient shear to disperse the powder throughout the solution.
Alternatively, existing solutions of pre-dissolved CMC having a high solids concentration generally produce high-viscosity solutions (e.g., greater than 5000 mPas, measured on a Brookfield RV at 100 rpm and 50-60 degrees Celsius) that are difficult to process on traditional equipment, e.g., in pumping or screening processes.
CMC also can be used as, or to make, a surface treatment, such as a barrier material. Barrier materials containing CMC can be oil and grease resistant, oxygen resistant, or both. Common film coating technologies used to form barrier materials in the shape of a film include a size press, a metering size press, and various curtain coating technologies. For each of these technologies, solution up-take is an important parameter that depends, at least in part, on the viscosity and/or solids content of the polymer solution that is used. Not wishing to be bound by any theory, it is believed that uniform barrier materials, especially films, result from a high enough coat weight, good film hold-out, or both. These properties may be more easily obtained by making a barrier material with CMC solutions of higher concentration. Currently, a higher concentration of CMC in solution is frequently achieved by degrading the CMC using various enzyme technologies. The enzymes, however, typically decrease the molecular weight of the CMC so drastically that the resulting barrier materials do not have very good film hold-out and penetrate into a base web.
Solutions previously have been proposed to provide stable, concentrated, aqueous CMC suspensions. For example, U.S. Pat. No. 4,883,537 to Burdick describes aqueous CMC solutions having decreased viscosity that include at least 33 weight percent potassium carbonate. However, the high concentration of potassium carbonate required limits the usefulness of these solutions in applications and makes these compositions undesirable for high CMC solids content applications.
Thus, there remains a need to provide a method for preparing low-viscosity, high CMC solids compositions. More particularly, there remains a need to develop CMC products that can be used to prepare solutions having higher solids content than normal paper coating grades and maintaining traditional CMC coating benefits while providing greater flexibility to prepare higher solids coatings using existing CMC equipment and makedown procedures.